1. Technical Field
The present invention relates, in general, to an improved method and system to be utilized with wireless communication systems having cellular architectures. In particular, the present invention relates to an improved method and system, to be utilized with wireless communication systems having cellular architectures, and which ensure near uniform capacity and quality of channels within each cell. Still more particularly, the present invention relates to an improved method and system, to be utilized with wireless communication systems having cellular architectures, and which ensure near uniform capacity and quality of channels within each cell by taking the measured received signal strength of mobile subscriber units into account when assigning relatively noisy and relatively noise free channels to mobile subscriber units either during handoff from other cells into a current cell or in response to a request for call access originating in a current cell.
2. Description of the Related Art
The present invention is related to wireless communication systems, and, in particular, to wireless communications systems having a cellular architecture (e.g., cellular telephony, Personal Communications Systems, or Global Systems for Mobil Communication). Wireless communication refers to the fact that transmission between sending and receiving stations occurs via electromagnetic radiation not guided by any hard physical path (e.g., by microwave link). Cellular architecture refers to the fact that the wireless system effects service over an area by utilizing a system that can be pictographically represented as a cellular grid.
Wireless cellular communication is the latest incarnation of a technology that was originally known as mobile telephone systems. Early mobile telephone system architecture was structured similar to television broadcasting. That is, one very powerful transmitter located at the highest spot in an area would broadcast in a very large radius. If a user were in the useable radius, then that user could broadcast to the base station and thus communicate by radiotelephone to the base station. However, such systems proved to be very expensive for the users and not very profitable to the communications companies maintaining such systems. The primary limiting factor of the original mobile telephone systems was that the number of channels available for use was limited due to severe channel-to-channel interference within the area served by the powerful transmitter. Thus, a problem arose as to how to provide more channels within the service area.
Counterintuitively, engineers discovered that channel-to-channel interference effects within the service area were not solely due to the distance between stations communicating with the base transmitter (which intuitively would seem to give rise to the interference), but were also inversely related to the transmitter power (radius) of the area being served by the transmitter. Engineers found that by reducing the radius of an area being served by fifty percent, service providers could increase the number of potential customers in an area fourfold. It was found that systems based on areas with a one-kilometer radius would have one hundred times more channels than systems with areas ten kilometers in radius. Speculation led to the conclusion that by reducing the radius of areas to a few hundred meters, the number of calls that could be served by each cell could be greatly increased.
Thus, reducing the power of the central transmitter allowed a significant increase in the number of available channels by reducing channel-to-channel interference within an area. However, as the power of the central transmitter was reduced, the serviceable area was also reduced. Thus, although reducing transmission power increased the number of available channels, the small service area provided by such reduced power did not make such radio telephone systems attractive communications options for many users. Thus, a problem arose relating to how to utilize the discovery that smaller cell sizes increased available channels in a fashion that would provide service attractive to users.
This problem was solved by the invention of the wireless cellular architecture concept. The wireless cellular architecture concept utilizes geographical subunits called "cells" and is buttressed by what is known as a frequency reuse concept. A cell is the basic geographic unit of a cellular system. Cells are base stations (a base station consists of hardware located at the defining location of a cell and includes power sources, interface equipment, radio frequency transmitters and receivers, and antenna systems) transmitting over small geographic areas that are represented as hexagons. Each cell size varies depending on the landscape. The term "cellular" comes from the honeycomb shape of the areas into which a coverage region is divided. Because of constraints imposed by natural terrain and man-made structures, the true shape of cells is not a perfect hexagon, but such shape serves as an effective tool for design engineering.
Within each cell a base station controller talks to many mobile subscriber units at once, using one defined transmit/receive communications channel per mobile subscriber unit. A mobile subscriber unit (a control unit and a transceiver that transmits and receives wireless transmissions to and from a cell site) uses a separate, temporary wireless channel to talk to a cell site. Transmit/receive communications channels use a pair of frequencies for communication--one for transmitting from the cell site base station controller, named the forward link, and one frequency for the cell site to receive calls from the users, named the reverse link. Both the forward and reverse link must have sufficient bandwidth to allow transmission of user data.
The frequency reuse concept is what made wireless cellular communications a viable reality. Wireless communication is regulated by government bodies (e.g., the Federal Communications Commission). Government bodies dictate what frequencies in the wireless spectrum can be utilized for particular applications. Consequently, there are is a finite set of frequencies available for use with wireless cellular communications. The frequency reuse concept is based on assigning to each cell a group of radio channels utilized within a small geographic area (cell). Adjacent cells are assigned a group of channels that is completely different from any neighboring cell. Thus, in the frequency reuse concept there is always a buffer cell between two cells utilizing the same set of frequencies. The cells are sized such that it is not likely that two cells utilizing the same set of frequencies will interfere with each other. Thus, such a scheme allows "frequency reuse" by non-adjacent cells.
Since each contiguous cell utilizes different frequencies, the ability for such a system to supply continuous service across a cell grid requires that a call-in-progress be switched to a new transmit/receive channel as a user transits from one cell into another. That is, since adjacent areas do not use the same wireless channels, a call must either be dropped or transferred from one wireless channel to another when a user crosses the line between adjacent cells. Because dropping the call is unacceptable, the process of "handoff" was created. Handoff occurs when the mobile telephone network automatically transfers a call from one wireless channel to another wireless channel as a mobile subscriber unit crosses adjacent cells.
Handoff works as follows. During a call, a moving mobile subscriber unit is utilizing one channel. When the mobile unit moves out of the coverage area of a given cell site, the reception becomes weak. At this point, the base station controller in use requests a handoff. The system switches the call to another different frequency channel in a new cell without interrupting the call or alerting the user. The call continues as long as the user is talking, and generally the user barely notices the handoff.
The foregoing ideas of cells, frequency reuse, and handoff constituted the invention of the cellular concept. The invention of the cellular concept made the idea of wireless cellular communications a viable commercial reality.
The first large scale wireless communication system utilizing cellular architecture in North America was the Advanced Mobile Phone Service (AMPS) which was released in 1983. AMPS utilizes the 800-MHz to 900-MHz frequency band and the 30 KHz bandwidth for each transmit/receive channel as a fully automated mobile telephone service. Designed for use in cities, AMPS later expanded to rural areas. It maximized the cellular concept of frequency reuse by reducing radio power output. AMPS is utilized throughout the world and is particularly popular in the United States, South America, China, and Australia. AMPS uses frequency modulation (FM) for radio transmission. In the United States, transmission between the mobile and the base station uses separate frequencies on the forward and reverse links.
With the introduction of AMPS, user demand for bandwidth was initially slow until users became acquainted with the power of such a system. However, once users became acquainted with the power of cellular, the demand for the service exploded. Very quickly, even the extended number of channels available utilizing the cellular concepts of reduced power output and frequency reuse were quickly consumed. Users demanded yet more bandwidth, and a problem arose in the cellular industry.
Engineers responded to the problem by devising the Narrowband Analog Mobile Phone Service (NAMPS). In this second generation of analog cellular systems, NAMPS was designed to solve the problem of low calling capacity. In the NAMPS three transmit/receive channels are frequency division multiplexed into the AMPS 30-kHz single transmit/receive channel bandwidth. Frequency division multiplexing is the process of deriving two or more simultaneous, continuous channels from a propagation medium that connects two points by (a) assigning separate portions of the available frequency spectrum to each of the individual channels, (b) dividing the frequency range into narrow bands, and (c) using each narrow band as a separate channel. Weik, Communications Standard Dictionary 375 (3ed. 1995). NAMPS services three users in one AMPS transmit/receive channel by dividing the 30-kHz AMPS bandwidth into three transmit/receive 10-kHz channels.
Thus, NAMPS essentially tripled the capacity of AMPS. However, although NAMPS multiplied the capacity of AMPS, it also introduced significant adjacent channel interference effects. Users did not find such interference acceptable, which created a problem. The problem now was how to maintain the extended capacity of the NAMPS system, but without the interference effects.
This problem was more difficult, because at this point the engineers had pushed the limits of the analog channels of AMPS, via NAMPS, to their absolute data carrying capacity limits. Since the spectrum available to cellular was now being utilized as efficiently as possible, engineers had to find a new way to increase the bandwidth of AMPS, but without the adjacent channel interference introduced by NAMPS. They accomplished this by the overlaying of digital multiplexing technologies onto the analog channels available in AMPS, and NAMPS. Such overlaying schemes are generally referred to as Digital AMPS, or DAMPS. North American digital cellular is alternatively referred to as both DAMPS and TDMA. One of the technologies so overlaid is that of Time Division Multiple Access (TDMA).
Whereas frequency division multiplexing divides a transmit/receive channel into narrow frequency band transmit/receive channels so that more user data can be sent in the original transmit/receive channel, TDMA uses digital techniques to divide time access to an analog channel before users are even allowed to access the analog channel. TDMA uses digital signals and provides each call with time slots into which to insert digital data, so that several calls can occupy one bandwidth. Each caller is assigned a specific time slot. In some cellular systems, digital packets of information are sent during each time slot and reassembled by the receiving equipment into original signal components. TDMA uses the same frequency band and channel allocations as AMPS and NAMPS. Thus, such technology has extended the usable bandwidth of the AMPS to that of NAMPS, but has done so without the adjacent channel interference that is a byproduct of NAMPS.
Like NAMPS, TDMA provides three channels (i.e., supports three mobile subscriber units) in the same bandwidth as a single AMPS channel (that is, the analog transmission portion of TDMA is very similar to that of NAMPS). Unlike NAMPS, in TDMA digital signal processing is utilized to compress the spectrum necessary to transmit information by compressing idle time and redundancy of messages to be sent over a channel. Once such compressed data has been sent over a channel, sister digital processing equipment on the other end of the channel decompresses the signal. Such compression effectively allows more users to communicate over the bandwidth of AMPS.
AMPS, NAMPS and TDMA are currently being utilized in many parts of the world. AMPS and NAMPS both utilize handoff. Furthermore, since TDMA is digital multiplexing overlaid onto either AMPS or NAMPS, TDMA also utilizes handoff.
Thus, AMPS, NAMPS, and TDMA all utilize cellular architecture and some variant of the above described handoff mechanism. For reasons that will now be described, the currently utilized methods of assigning channels to mobile subscriber units during handoff are deficient in that they fail to utilize existing information in order to ensure near uniform capacity and quality of channels within a cell.
Within each cell, there appears a range of what can be characterized as relatively noisy channels to what can be characterized as relatively noise-free channels. Ordinarily, when a mobile subscriber unit transits from one cell to another cell, a channel selection algorithm inside the cell into which the mobile subscriber unit has transited assigns the mobile subscriber unit a channel on which to communicate while in the cell. The same channel selection algorithm also tends to be utilized to assign channels to mobile subscriber units in response to requests for channel access which are not handoff requests (e.g., requests for channel access originating internal to a cell). At present, these channel selection algorithms attempt to provide clear communications for users by assigning, on a first come first served basis, relatively noise-free channels prior to assigning relatively noisy channels.
The practice of assigning the relatively noise-free channels on a first come first served basis is deficient. It is deficient in that it fails to take into account the fact that certain mobile subscriber units can tolerate a relatively noisy channel much more easily than other mobile subscriber units. For example, mobile subscriber units vary in power (e.g., a mobile telephone typically has a transmit power of 3.0 watts, while a portable typically has a transmit power of 0.6 watts, while a transportable typically has a transmit power of 1.6 watts). Thus, all other things being equal, it would make more sense to assign the higher power transmitters to the more noisy channels, since the signal-to-noise ratios of such an assignment would be better than if lower power transmitters were assigned to the relatively noisy channels.
In addition, assigning mobile units whose signal strengths are higher to more noisy channels makes good sense in that the objective of the cell is to maintain a certain minimum transmission capacity on all channels utilized within the cells. As has been discussed, TDMA is now utilizing transmission of digital data. The signal-to-noise ratio is especially important in the transmission of digital data, in that it determines the upper bound on the achievable data rate. A result from information theory is that the maximum channel capacity, in bits per second, obeys the equation C=W log.sub.2 (1+S/N) where W is the bandwidth of the channel in hertz and S/N is the signal-to-noise power ratio express in dBs. Thus, in order to maintain similar capacities on all channels it makes sense to strive for similar signal-to-noise ratios on all channels.
Thus, it is apparent that a need exists for a method and system that takes the measured received signal strength of mobile subscriber units into account when allocating relatively noisy to relatively noise free channels to mobile subscriber units when such allocation is done in response to mobile subscriber units' request for channel access, irrespective of whether such request is to service a handoff request or a call access request.